ARM: pxa: fix pxa25x interrupt init

In the device-tree case, the root interrupt controller cannot be
accessed through the 6th coprocessor, contrary to pxa27x and pxa3xx
architectures.

Fix it to behave as in non-devicetree builds.

Fixes: 32f17997 ("ARM: pxa: remove irq init from dt machines")
Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>